Determining battery dc impedance

ABSTRACT

A method and apparatus for measuring battery cell DC impedance by controlling charging of the battery cell. The method includes real-time characterization of a battery, (a) measuring periodically a DC impedance of the battery to determine a measured DC impedance; (b) ratioing the measured DC impedance to a reference DC impedance for the battery to establish an impedance degradation factor; (c) obtaining, during use of the battery and responsive to a set of attributes of the battery, an operational reference impedance for the battery; and (d) applying the impedance degradation factor to the operational reference impedance to obtain a real-time effective impedance for the battery.

BACKGROUND OF THE INVENTION

The present invention relates generally to estimations of health forbatteries used in heavy power use applications, and more particularly toa state of health for electric vehicle and hybrid vehicle batteries andbattery packs.

It is important for users and manufacturers of vehicles using batteriesthat the health and performance of those batteries be monitored.Batteries lose power and capacity (the specific mechanisms of that lossvary based upon cell chemistry) therefore it is important that the powerand capacity be known to aid in service diagnostics and power limitalgorithms during usage. The following description is specificallyfocused on lithium-ion chemistry but other chemistries may be benefitedfrom the following description.

Current techniques for monitoring the battery health include batterycapacity measurements and estimates. This is an important batteryparameter, but it is the case that a battery having sufficient capacitymay cause a user, under certain conditions, to experience a sudden lossin available power from the battery.

One way to estimate available power is based upon measurement of ACimpedances of the batteries, using conventional techniques such asKalman filtering. Knowledge of AC impedance allows accurate real-time DCpower estimation. However, since the real-time impedance is a functionof state of charge and health, this estimation does not indicate therelative degradation in power to a fresh pack.

A current method for estimating available power uses a look-up table.The table uses information about temperature, state of charge (SOC), andbattery age to predict and estimate available power. This predictivemethod fails to account for varying degrees of degradations in batterychemistry that occur over long periods that result from varyingoperating environments. For example, a user operating an electricvehicle in a hot climate may experience shorter battery life due to hightemperature usage.

What is needed is an apparatus and method to measure battery degradationin contrast to conventional techniques of predicting the batterydegradation.

BRIEF SUMMARY OF THE INVENTION

Disclosed is a method and apparatus for measuring battery cell DCimpedance by controlling charging of the battery cell. The methodincludes real-time characterization of a battery, (a) measuringperiodically a DC impedance of the battery to determine a measured DCimpedance; (b) ratioing the measured DC impedance to a reference DCimpedance for the battery to establish an impedance degradation factor;(c) obtaining, during use of the battery and responsive to a set ofattributes of the battery, an operational reference impedance for thebattery; (d) applying the impedance degradation factor to theoperational reference impedance to obtain a real-time effectiveimpedance for the battery; and (e) managing the battery temperature to awarm temperature so that the user does not notice a delay in chargingtime when measuring the DC impedance.

The apparatus includes a battery charging system for a battery having acharger coupled to the battery, a controller, and a battery dataacquisition and monitoring subsystem wherein the controller: measuresperiodically a DC impedance of the battery to determine a measured DCimpedance; ratios the measured DC impedance to a reference DC impedancefor the battery to establish an impedance degradation factor; obtains,during use of the battery and responsive to a set of attributes of thebattery acquired by the battery data acquisition and monitoringsubsystem, an operational reference impedance for the battery; andapplies the impedance degradation factor to the operational referenceimpedance to obtain a real-time effective impedance for the battery.Various functions and structures of this system may be divided orintegrated together into different elements than described herein.

Embodiments of the present invention provide apparatus and method tomeasure battery degradation directly. Knowing both available power andcurrent capacity helps the operator of an apparatus, like an electricvehicle, avoid dangerous uses of the apparatus that they may beotherwise able to avoid should they have a better representation ofavailable power. In terms of manufacturers and maintenance, accurateimpedance measurement, in cooperation with battery capacity, provides amore reliable indicator of the state of health of the battery ascompared to battery capacity alone.

Knowing the available power is important in other aspects of use of thebattery. For electric vehicles, there is a specification regardingavailable sustained peak power which is directly related to availablepower. In application of an electric vehicle, for those without theembodiments of the present invention, it may be the case that a userinitiates a maneuver with increased risk (e.g., passing another vehicle)and the duration of the sustained power peak cannot be maintained forthe expected duration. The user experiences what appears to be a suddenloss in power which can have many different consequences depending uponthe situation and how the user reacts. Embodiments of the presentinvention may be used as a feed-forward control path to pre-restrictoperation to power levels that can be sustained for specified periods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative charging system;

FIG. 2 is a control diagram for the charging system shown in FIG. 1;

FIG. 3 is a second control diagram for the charging system shown in FIG.1 for dampening effects of noise, temperature variation, and measurementinaccuracies during the process described in FIG. 4; and

FIG. 4 is a process diagram for the charging system shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method formeasuring battery degradation in contrast to conventional techniques ofpredicting the battery degradation, particularly for lithium-ion batterycells. The following description is presented to enable one of ordinaryskill in the art to make and use the invention and is provided in thecontext of a patent application and its requirements. Variousmodifications to the preferred embodiment and the generic principles andfeatures described herein will be readily apparent to those skilled inthe art. Thus, the present invention is not intended to be limited tothe embodiment shown but is to be accorded the widest scope consistentwith the principles and features described herein.

FIG. 1 is a preferred embodiment for a charging system 100, such as maybe used in an electric vehicle. System 100 includes a battery 105, acharger 110 coupled to battery 105 and a battery management system (BMS)115 and a battery data acquisition and monitoring subsystem 120. Acommunication bus 125 couples subsystem 120 to BMS 115 and acommunication bus 130 couples BMS 115 to charger 110. A communicationbus 135 couples battery data from battery 105 to subsystem 120.

Battery 105 is shown as a series-connected group of battery cells,however the arrangement of cells may be a combination of parallel/seriesconnected cells of many different arrangements. Charger 110 of thepreferred embodiment provides the charging current applied to battery105. BMS 115 controls the charging current according to a profileestablished by the embodiments of the present invention. Subsystem 120acquires the desired data as described herein regarding battery 105. Forexample, voltage, SOC, temperature, and other applicable data used byBMS 115. In some embodiments, subsystem 120 may be part of BMS 115 andBMS 115 may be part of charger 110. One or more of charger 110, BMS 115,and subsystem 120 control a switch 140. Again, the organization,arrangement, and distribution of the functions described herein mayprovided other than as described in this exemplary embodiment.

FIG. 2 is a control diagram 200 for the charging system shown in FIG. 1.Diagram 200 describes a typical control system as may be used forcharging lithium ion cells. A target voltage 205 and a maximum cellvoltage 210 are subtracted and used by a controller 215 to produce acharging current 220. In prior art systems, current 220 is constant orcompensates for an internal resistance (IR) drop of battery 105. Asdescribed above, the preferred embodiments of the present inventiondescribe a measurement system for actual DC impedance. In broad terms,the measurement of real-time DC impedance permits calculation ofavailable current/available power which may be used in a variety of waysincluding representing a state-of-health of a battery and providing afeed-forward parameter that may be used to indicate/control a currentlevel of sustained peak power.

Lithium ion batteries are common in consumer electronics. They are oneof the most popular types of battery for portable electronics, with oneof the best energy-to-weight ratios, no memory effect, and a slow lossof charge when not in use. In addition to uses for consumer electronics,lithium-ion batteries are growing in popularity for defense, automotive,and aerospace applications due to their high energy and power density.However, certain kinds of treatment may cause Li-ion batteries to failin potentially dangerous ways.

One of the advantages of use of a Li-ion chemistry is that batteriesmade using this technology are rechargeable. Traditional charging isdone with a two-step charge algorithm: (i) constant current (CC), and(ii) constant voltage (CV). In electric vehicles (EVs), the first stepcould be constant power (CP).

Step 1: Apply charging current limit until the volt limit per cell isreached.

Step 2: Apply maximum volt per cell limit until the current declinesbelow a predetermined level (often C/20 but sometimes C/5 or C/10 orother value).

The charge time is approximately 1-5 hours depending upon application.Generally cell phone type of batteries can be charged at 1 C, laptoptypes 0.8 C. The charging typically is halted when the current goesbelow C/10. Some fast chargers stop before step 2 starts and claim thebattery is ready at about a 70% charge. (As used herein, “C” is a ratedcurrent that discharges the battery in one hour.)

Generally for consumer electronics, lithium-ion is charged withapproximate 4.2±0.05 V/cell. Heavy automotive, industrial, and militaryapplication may use lower voltages to extend battery life. Manyprotection circuits cut off when either >4.3 V or 90° C. is reached.

Battery chargers for charging lithium-ion-type batteries are known inthe art. As is known in the art, such lithium ion batteries requireconstant current (CC) and constant voltage (CV) charging. In particular,initially such lithium ion batteries are charged with a constantcurrent. In the constant current mode, the charging voltage is typicallyset to a maximum level recommended by the Li-ion cell manufacturer basedon safety considerations, typically 4.2V per cell. The charging currentis a factor of design level, based on the cell capability, charge time,needs and cost. Once the battery cell voltage rises sufficiently, thecharging current drops below the initial charge current level. Inparticular, when the battery cell voltage Vb approaches the chargingvoltage Vc, the charging current tapers according to the formula:I=(Vc−Vb)/Rs, where I=the charging current, Vc=the charging voltage,Vb=the battery cell open circuit voltage and Rs=the resistance of thecharging circuit including the contact resistance and the internalresistance of the battery cell. As such, during the last portion of thecharging cycle, typically about the last ⅓, the battery cell is chargedat a reduced charging current, which means it takes more time to fullycharge the battery cell.

The closed-circuit voltage represents the voltage of the battery cellplus the voltage drops in the circuit as a result of resistance in thebattery circuit, such as the battery terminals and the internalresistance of the battery cell. By subtracting the closed-circuitvoltage from the open-circuit voltage, the voltage drop across thebattery resistance circuit elements can be determined.

FIG. 3 is a second control diagram for the charging system shown in FIG.1 for dampening effects of noise, temperature variation, and measurementinaccuracies during the process described in FIG. 4 below. The controlis an implementation of a low-pass filter to help reduce variations fromthe noise, temperature variation, and measurement inaccuracies.

FIG. 4 is a diagram for a process 400 implemented by charging system 100shown in FIG. 1. Process 400 calculates power or current available. Thecalculation shown is described for individual cells of a multi-cellbattery pack. For pack power or pack current, all cell values aresummed. Process 400 is implemented every one to four weeks when thebattery is being charged (preferably at night) and when the battery isabout sixty percent state-of-charge (SOC). For the discussion below, theterm “battery” is used to simplify the discussion. In the preferredimplementation, the battery is a multi-cell pack and when the term“battery’ is used, it may be exchanged for cell and applicable to eachcell of the battery pack, unless the context implies differently.

Process 400 begins with step 405 to set battery temperature. This stepwarms the battery and keeps the battery (cells) at a desiredtemperature, in the preferred embodiment this temperature isapproximately thirty-five degrees Celsius. This may be managed inmultiple different ways, for example based upon coolant flow from theprevious drive cycle. Air and/or liquid or other cooling mechanism isused to equalize cell temperature at the desired level before proceedingwith process 400.

After step 405, process 400 charges the battery in desired fashion untilthe average SOC of the battery reaches about a fixed predeterminedpercent SOC (typically around sixty percent SOC). The particular valuechosen is based upon application and other design considerations. Thereare some trade-offs in selecting the desired SOC for this step. Between40% to 100% SOC DC impedance of a battery tends to be flat. Also, forCobalt cells, the OCV/SOC curve is flat at values <55% SOC. It is thecase that various applications and usage patterns will help determinesome of these values. For example, some patterns of usage may have usersdischarge their batteries further than other applications beforerecharging. It is desirable to base the threshold on typical userdischarge patterns. In some applications, a value for SOC is chosen >60%when the SOC is accurate and the impedance is flat and users typicallyoperate their vehicle into this SOC. For longer range batteries, someusers may only drive the vehicle down to 70% SOC, in which case theselected value of 70% may be more desired. There are disadvantages tochoosing too high of an SOC. For example, the battery may start hittingthe taper voltage.

Step 410 has charging system 100 command zero amps from the charger tothe battery for a relaxation period. The relaxation period of thepreferred implementation is on the order of about five minutes. Thisrelaxation period permits the battery to depolarize. The actual periodmay vary from five minutes, especially as the battery ages or variesfrom the desired temperature or when higher charge currents are used.

After the conclusion of the relaxation period of step 410, process 400performs step 415 of storing relaxed parameters. In the preferredembodiment, the relaxed battery voltage is measured and stored.

Next, after step 415, process 400 performs step 420 and resumesfull-current charging. In the preferred embodiment, the charge currentis desirably at least about C/3. This level increases accuracy of the DCimpedance measurement reducing cell voltage/current measurement errors.

Following step 420, process 400 performs step 425 to sample cellvoltages. Preferably, step 425 is implemented after a sustained peakpower period lapses. For many electric vehicles in conventional usage,it is determined that the sustained peak power period should be on theorder of about ten seconds. For some applications, it may be that thesustained peak power be eighteen seconds. This value is determined bythe application of the battery. Step 430, following step 425, calculatesan impedance for each cell. The impedance R is the change in voltagedivided by the change in current (ΔV/ΔI).

The preferred implementation uses the low pass filter shown in FIG. 3 tomeasure the impedance of each cell. Other implementations of the lowpass filter may be desirable, depending upon a variety of factors.

Once the measured impedance for each cell is calculated, process 400performs step 435 to determine a state-of-health (SoH) impedancedegradation factor. This factor is a ratio of the measured impedancefrom step 430 to a “reference” impedance that represents the impedanceof a fresh, newly manufactured battery. This ratio will be a value lessthan 1.0.

During operation (e.g., while driving an electric vehicle using thebattery), process 440 (at step 440) determines a reference impedance forthe battery. Preferably step 440 uses a look-up table that is responsiveto SoC and battery temperature to establish the reference impedance.Next process 400 performs step 445 to calculate the real DC impedance.The real DC impedance equals the SOH factor from step 435 multipliedtimes the reference impedance. This real-time DC impedance value may beused for different purposes. It is used in step 450 of process 400 ofthe preferred embodiment to calculate the available power/current of thebattery. (For power, the calculation represents an individual cell sothat all cell power limits are summed to get a total power available forthe battery pack.) In the following, DchILimit is the Discharge CurrentLimit, V_(min) is a value determined by the manufacturer for theparticular application, V_(cell) is the actual voltage level of thebattery, and R is the real-time DC impedance value from step 445.V_(min) is typically around three volts for consumer batteries and abouttwo and seven tenths volts for electric vehicles. V_(max) is amanufacturer determined maximum cell voltage. V_(min) and V_(max) aretypically constant but may vary by temperature. Embodiments of thepresent invention may use a dynamic value for these voltages whennecessary or desirable.

DchILimit=(V _(min) −V _(Cell))/R

DchPLimit=DchILimit*V _(min)

ChgILimit=(V _(max) −V _(Cell))/R

ChgPLimit=ChgILimit*V _(max)

The system above has been described in the preferred embodiment of anembedded automobile (EV) electric charging system. The system, method,and computer program product described in this application may, ofcourse, be embodied in hardware; e.g., within or coupled to a CentralProcessing Unit (“CPU”), microprocessor, microcontroller, digital signalprocessor, System on Chip (“SOC”), or any other programmable device.Additionally, the system, method, and computer program product, may beembodied in software (e.g., computer readable code, program code,instructions and/or data disposed in any form, such as source, object ormachine language) disposed, for example, in a computer usable (e.g.,readable) medium configured to store the software. Such software enablesthe function, fabrication, modeling, simulation, description and/ortesting of the apparatus and processes described herein. For example,this can be accomplished through the use of general programminglanguages (e.g., C, C++), GDSII databases, hardware descriptionlanguages (HDL) including Verilog HDL, VHDL, AHDL (Altera HDL) and soon, or other available programs, databases, nanoprocessing, and/orcircuit (i.e., schematic) capture tools. Such software can be disposedin any known computer usable medium including semiconductor (Flash, orEEPROM, ROM), magnetic disk, optical disc (e.g., CD-ROM, DVD-ROM, etc.)and as a computer data signal embodied in a computer usable (e.g.,readable) transmission medium (e.g., carrier wave or any other mediumincluding digital, optical, or analog-based medium). As such, thesoftware can be transmitted over communication networks including theInternet and intranets. A system, method, computer program product, andpropagated signal embodied in software may be included in asemiconductor intellectual property core (e.g., embodied in HDL) andtransformed to hardware in the production of integrated circuits.Additionally, a system, method, computer program product, and propagatedsignal as described herein may be embodied as a combination of hardwareand software.

One of the preferred implementations of the present invention is as aroutine in an operating system made up of programming steps orinstructions resident in a memory of a computing system as well known,during computer operations. Until required by the computer system, theprogram instructions may be stored in another readable medium, e.g. in adisk drive, or in a removable memory, such as an optical disk for use ina CD ROM computer input or other portable memory system for use intransferring the programming steps into an embedded memory used in thecharger. Further, the program instructions may be stored in the memoryof another computer prior to use in the system of the present inventionand transmitted over a LAN or a WAN, such as the Internet, when requiredby the user of the present invention. One skilled in the art shouldappreciate that the processes controlling the present invention arecapable of being distributed in the form of computer readable media in avariety of forms.

Any suitable programming language can be used to implement the routinesof the present invention including C, C++, Java, assembly language, etc.Different programming techniques can be employed such as procedural orobject oriented. The routines can execute on a single processing deviceor multiple processors. Although the steps, operations or computationsmay be presented in a specific order, this order may be changed indifferent embodiments. In some embodiments, multiple steps shown assequential in this specification can be performed at the same time. Thesequence of operations described herein can be interrupted, suspended,or otherwise controlled by another process, such as an operating system,kernel, and the like. The routines can operate in an operating systemenvironment or as stand-alone routines occupying all, or a substantialpart, of the system processing.

In the description herein, numerous specific details are provided, suchas examples of components and/or methods, to provide a thoroughunderstanding of embodiments of the present invention. One skilled inthe relevant art will recognize, however, that an embodiment of theinvention can be practiced without one or more of the specific details,or with other apparatus, systems, assemblies, methods, components,materials, parts, and/or the like. In other instances, well-knownstructures, materials, or operations are not specifically shown ordescribed in detail to avoid obscuring aspects of embodiments of thepresent invention.

A “computer-readable medium” for purposes of embodiments of the presentinvention may be any medium that can contain, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, system or device. The computerreadable medium can be, by way of example only but not by limitation, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, system, device, propagation medium, orcomputer memory.

A “processor” or “process” includes any human, hardware and/or softwaresystem, mechanism or component that processes data, signals or otherinformation. A processor can include a system with a general-purposecentral processing unit, multiple processing units, dedicated circuitryfor achieving functionality, or other systems. Processing need not belimited to a geographic location, or have temporal limitations. Forexample, a processor can perform its functions in “real time,”“offline,” in a “batch mode,” etc. Portions of processing can beperformed at different times and at different locations, by different(or the same) processing systems.

Reference throughout this specification to “one embodiment”, “anembodiment”, or “a specific embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention and notnecessarily in all embodiments. Thus, respective appearances of thephrases “in one embodiment”, “in an embodiment”, or “in a specificembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics of any specificembodiment of the present invention may be combined in any suitablemanner with one or more other embodiments. It is to be understood thatother variations and modifications of the embodiments of the presentinvention described and illustrated herein are possible in light of theteachings herein and are to be considered as part of the spirit andscope of the present invention.

Embodiments of the invention may be implemented by using a programmedgeneral purpose digital computer, by using application specificintegrated circuits, programmable logic devices, field programmable gatearrays, optical, chemical, biological, quantum or nanoengineeredsystems, components and mechanisms may be used. In general, thefunctions of the present invention can be achieved by any means as isknown in the art. Distributed, or networked systems, components andcircuits can be used. Communication, or transfer, of data may be wired,wireless, or by any other means.

It will also be appreciated that one or more of the elements depicted inthe drawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application. It isalso within the spirit and scope of the present invention to implement aprogram or code that can be stored in a machine-readable medium topermit a computer to perform any of the methods described above.

Additionally, any signal arrows in the drawings/Figures should beconsidered only as exemplary, and not limiting, unless otherwisespecifically noted. Furthermore, the term “or” as used herein isgenerally intended to mean “and/or” unless otherwise indicated.Combinations of components or steps will also be considered as beingnoted, where terminology is foreseen as rendering the ability toseparate or combine is unclear.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The foregoing description of illustrated embodiments of the presentinvention, including what is described in the Abstract, is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed herein. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes only, variousequivalent modifications are possible within the spirit and scope of thepresent invention, as those skilled in the relevant art will recognizeand appreciate. As indicated, these modifications may be made to thepresent invention in light of the foregoing description of illustratedembodiments of the present invention and are to be included within thespirit and scope of the present invention.

Thus, while the present invention has been described herein withreference to particular embodiments thereof, a latitude of modification,various changes and substitutions are intended in the foregoingdisclosures, and it will be appreciated that in some instances somefeatures of embodiments of the invention will be employed without acorresponding use of other features without departing from the scope andspirit of the invention as set forth. Therefore, many modifications maybe made to adapt a particular situation or material to the essentialscope and spirit of the present invention. It is intended that theinvention not be limited to the particular terms used in followingclaims and/or to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include any and all embodiments and equivalents falling within thescope of the appended claims. Thus, the scope of the invention is to bedetermined solely by the appended claims.

1. A method for real-time characterization of a battery, the methodcomprising: (a) measuring periodically a DC impedance of the battery todetermine a measured DC impedance; (b) ratioing said measured DCimpedance to a reference DC impedance for the battery to establish animpedance degradation factor; (c) obtaining, during use of the batteryand responsive to a set of attributes of the battery, an operationalreference impedance for the battery; and (d) applying said impedancedegradation factor to said operational reference impedance to obtain areal-time effective impedance for the battery.
 2. The method of claim 1wherein said measuring step (a) is implemented no more frequently thanabout once every week during a charging cycle for the battery when thebattery has a state of charge below about sixty percent.
 3. The methodof claim 1 wherein said measuring step (a) comprises: (a1) establishinga temperature of the battery at a desired temperature; (a2) charging thebattery to a predetermined SOC; (a3) applying zero amps charging currentto the battery for a relaxation period; (a4) measuring and storing cellvoltage of the battery at an end of said relaxation period; (a5)resuming charging at full charging current; (a6) determining, after asustained peak power or peak current period after resuming charging, acharging cell voltage for the battery; (a7) establishing said measuredimpedance for the battery by using changes of a battery voltage and abattery current from an expiration of said relaxation period and anexpiration of said sustained peak power period.
 4. The method of claim 3wherein said predetermined SOC is in the range of about sixty percent toabout seventy percent.
 5. The method of claim 1 wherein said set ofattributes include one or more attributes selected from the groupconsisting of state of charge, temperature, and combinations thereof. 6.The method of claim 5 wherein said obtaining step (c) employs abattery-in-use look-up table responsive to said set of attributes. 7.The method of claim 1 further comprising: (e) calculating a currentdischarge limit for the battery by subtracting V_(cell) from V_(min) andthen dividing by said real-time effective impedance.
 8. The method ofclaim 7 further comprising: (f) calculating a power discharge limit bymultiplying said current discharge limit by V_(min).
 9. A batterycharging system for a battery, comprising: a charger coupled to thebattery, a controller, and a battery data acquisition and monitoringsubsystem wherein said controller: measures periodically a DC impedanceof the battery to determine a measured DC impedance; ratios saidmeasured DC impedance to a reference DC impedance for the battery toestablish an impedance degradation factor; obtains, during use of thebattery and responsive to a set of attributes of the battery acquired bysaid battery data acquisition and monitoring subsystem, an operationalreference impedance for the battery; and applies said impedancedegradation factor to said operational reference impedance to obtain areal-time effective impedance for the battery.
 10. A computer programproduct comprising a computer readable medium carrying programinstructions for operating a system when executed using a computingsystem, the executed program instructions executing a method, the methodcomprising: (a) measuring periodically a DC impedance of the battery todetermine a measured DC impedance; (b) ratioing said measured DCimpedance to a reference DC impedance for the battery to establish animpedance degradation factor; (c) obtaining, during use of the batteryand responsive to a set of attributes of the battery, an operationalreference impedance for the battery; and (d) applying said impedancedegradation factor to said operational reference impedance to obtain areal-time effective impedance for the battery.
 11. The computer programproduct of claim 10 wherein said measuring step (a) is implemented nomore frequently than about once every week during a charging cycle forthe battery when the battery has a state of charge below about fiftypercent.
 12. The computer program product of claim 10 wherein saidmeasuring step (a) comprises: (a1) establishing a temperature of thebattery at a desired temperature; (a2) charging the battery to apredetermined SOC; (a3) applying zero amps charging current to thebattery for a relaxation period; (a4) measuring and storing cell voltageof the battery at an end of said relaxation period; (a5) resumingcharging at full charging current; (a6) determining, after a sustainedpeak power period after resuming charging, a charging cell voltage forthe battery; (a7) establishing said measured impedance for the batteryby using changes of a battery voltage and a battery current from anexpiration of said relaxation period and an expiration of said sustainedpeak power period.
 13. The computer program product of claim 12 whereinsaid predetermined SOC is in the range of about sixty percent to aboutseventy percent.
 14. The computer program product of claim 10 whereinsaid set of attributes include one or more attributes selected from thegroup consisting of state of charge, temperature, and combinationsthereof.
 15. The computer program product of claim 14 wherein saidobtaining step (c) employs a battery-in-use look-up table responsive tosaid set of attributes.
 16. The computer program product of claim 10further comprising: (e) calculating a current discharge limit for thebattery by subtracting V_(cell) from V_(min) and then dividing by saidreal-time effective impedance.
 17. The computer program product of claim16 further comprising: (f) calculating a power discharge limit bymultiplying said current discharge limit by V_(min).